This invention relates to a method for manufacturing dielectric ceramic compositions of high dielectric constant, and more particularly to a manufacturing method of the material for capacitors especially suitable for multilayer ceramic capacitors.
Along with the trend of miniaturization, electronic components are being made in chip-type structure. Requirements for multilayer ceramic chip capacitors, in particular, have become more stringent in reliability, compactness, large capacity, low cost, and small temperature dependency of capacitance and high insulating resistance. In the prior art, such requirements were met by mainly using ceramics of barium titanate (BaTiO.sub.3) group and shifting the Curie point by adding an additive or substituting the composition, in part, so as to lower the temperature dependency. Recently, attention has been focused on dielectrics having a composition of lead-based perovskite structure and par containing lead magnesium tungstate [Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 ] as disclosed in U.S. Pat. No. 4,450,240 granted on May 22, 1984 for Haruhiko Miyamoto et. al.
Barium titanate (BaTiO.sub.3) group is excellent in temperature characteristics, but its dielectric constant is approximately 3000 at maximum which cannot satisfy the stringent requirements for small multilayer ceramic chip capacitors. The perovskite structure compound mainly comprising lead magnesium tungstate [Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 ] is antiferroelectric at a temperature below the phase transition temperature (Curie temperature), and as a solid solution, is high in dielectric constant and insulating resistance and small in temperature variation.
However, following problems arise if the conventional method of synthesizing lead magnesium tungstate [Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 ] is attempted in which powders of lead oxide (PbO), magnesium oxide (MgO), and tungsten oxide are respectively mixed, calcined, pulverized and sintered. In this group, the reaction of lead oxide and tungsten oxide takes place preferentially at relatively low temperatures to produce Pb.sub.2 WO.sub.5 and compounds of unfixed ratios. These compounds are in a liquid phase at temperatures of 750.degree..about.850.degree. C. which is the conventional range of calcination, and tend to agglutinate at the time of calcination to prevent formation of a uniform composition. Magnesium oxide is relatively stable in this group, reacts weakly, and often remains in the sintered final product. As the liquid phase takes place, if calcination temperature is elevated to promote magnesium oxide reaction, powders become agglutinated and caked simultaneously. Similar problems are encountered in the case of perovskite solid solution containing as a component, lead magnesium tungstate. More specifically, when it is used in the manufacture of multilayer ceramic capacitors, the magnesium oxide reaction is not entirely complete, leaving unreacted residue within the sintered product after the sintering process. At the same time, excess lead oxide and tungsten oxide form a liquid phase an agglutinate at the grain boundary or at the triple point. Therefore, the dielectric constant of the sintered products becomes low, the temperature characteristics of the dielectric constant extremely unstable, and the resistivity and breakdown voltage reduced.